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“Using cannabis just once a week harms young brains,” the Daily Mail reports.

The newspaper reports on an US study that took one-off brain MRI scans of a group of 20 young adult recreational cannabis users, and a comparison group of 20 non-users. They compared their brain structure, focusing on regions that are believed to be involved in addiction.

They found differences between users and non-users in shape and volume of the nucleus accumbens and amygdala; areas of the brain involved in reward and pleasure responses, emotions, memory, learning, and decision making.

However, a case could be made that the media has overstated the implications of the research.

As the study only involved a single one-off brain scan it cannot prove cause and effect. It could be the case that pre-existing abnormalities in the brain make people more likely to use cannabis rather than vice versa.

The study was small, involving just 20 users and 20 non-users. Examining different groups of people and different age groups could give different results.

And finally, there is currently no proof that the changes detected to the brain will correspond to any demonstrable differences in thought processes and decision making behaviour.

That said, due to the widespread use of cannabis, results such as these warrant further study. This may possibly become easier to carry out due to the quasi-legal status of cannabis in some US states.

Where did the story come from?

The study was carried out by researchers from Massachusetts General Hospital, Harvard Medical School, Boston, and Northwestern University Feinberg School of Medicine, Chicago.

Funding was provided by the National Institute on Drug Abuse, the Office of National Drug Control Policy, Counterdrug Technology Assessment Center, the National Institute of Neurological Disorders and Stroke, and the National Institutes of Health. Individual researchers also received support from Warren Wright Adolescent Center at Northwestern Memorial Hospital and Northwestern University; and a Harvard Medical School Norman E. Zinberg Fellowship in Addiction Psychiatry Research.

The study was published in The Journal of Neuroscience, a peer-reviewed medical journal.

By and large the media has made the (potentially incorrect) assumption that cannabis use has harmed the brain and is responsible for alleged changes in behaviour. For example, the Daily Mail headline that “cannabis once a week harms young brains” in particular is not justified by this research.

The study did not investigate whether the brain changes observed were harmful (for example in terms of thinking or behaviour), they just commented that the brain structures were different. Also, users in the study averaged 11 cannabis joints per week, rather than one.

This small cross sectional study taking one-off brain scans cannot prove whether cannabis was behind any changes seen to the brain. Observational studies that followed people over time would be able to provide better evidence of this.

What kind of research was this?

This cross sectional study took MRI scans of the brains of young adults who used marijuana (cannabis) recreationally, and compared them with brain images of adults who did not use cannabis. They were interested in comparing the structure in particular areas of the brain.

Cannabis is one of the most commonly used illicit drugs, particularly by adolescents and young adults. It has been shown to have effects upon thought processes such as learning, memory, attention and decision-making.

Previous animal studies have shown that exposing rats to 9-tetrahydrocannabinol (THC), the main psychoactive chemical of cannabis, leads to changes in the structures including the nucleus accumbens. In people the nucleus accumbens is believed to play a central role in the brain’s reward centre and pleasure-seeking behaviour. However, less is known about the relationship between cannabis use and brain structure in people, and this is what this study aimed to look at.

What did the research involve?

The study included 20 young adults (aged 18–25 years; 9 male) current cannabis users and 20 controls who did not use cannabis. The controls were matched by age, sex, ethnicity, hand dominance and educational level. Cannabis users used cannabis at least once a week but were not considered to be dependent (as assessed using valid diagnostic criteria). They did not include people who met criteria for abuse of alcohol or any other substance.

The participants received MRI imaging on one visit to the study centre. They were asked not to use cannabis on that day. They performed a urine screen for any substance. The main breakdown product of THC can be detected in the urine several weeks after last use, so they couldn’t tell from the urine test how long ago participants had last used. But researchers checked that none showed signs of acute intoxication according to criteria on examination (for example fast heart rate, red eyes, slurred speech).

All participants were scanned using special MRI techniques, specifically looking at the volume, shape and density of gray matter (nerve cell bodies) in the nucleus accumbens and other brain regions that may be involved in addiction.

What were the basic results?

The researchers found that the gray matter of cannabis users was denser in the left nucleus accumbens, and in other brain regions including the amygdala, a region believed to play an important role in our emotional responses, including fear and pleasure. Correlating with the increased density of nerve cells, the volume of the left nucleus accumbens was also larger in cannabis users than non-users.

The higher the reported use of cannabis, the higher the volume of the left nucleus accumbens tended to be, and the greater the density of gray matter.

Cannabis users and non-users also demonstrated differences in brain shape, particularly in the left nucleus accumbens and right amygdala.

The observed differences were seen even after adjusting for age, sex, alcohol and cigarette use.

How did the researchers interpret the results?

The researchers conclude that their study suggests that cannabis use in young recreational users is associated with exposure-dependent alterations in the structure of the core brain regions involved in the reward system.

Conclusion

This study found differences between young recreational cannabis users and non-users in the volume and structure of the nucleus accumbens and amygdala, which have a role in the brain’s reward system, pleasure response, emotion and decision making.

However, as this was only a cross sectional study taking one-off brain scans of cannabis users and non-users, it cannot prove that cannabis use was the cause of any of the differences seen. It is not known whether cannabis use could have caused these changes in regular users.

Or conversely whether the cannabis users in this study had this brain structure to start with, and that this may have made them more likely to become regular users of cannabis.

Also, this is a small study comparing the brain structure of only 20 users and 20 non-users. With such a small sample of people, it is possible that any differences in brain structure could have been due to chance. These changes may not have been evident had a larger number of people been examined.

Examination of different samples of people, and in different age groups, may have given different results.

Similarly, examining the extent of brain structural change was related to factors such as age at first use, and frequency or duration of use, are less reliable when based on such a small sample of people.

Confirmation of these tentative findings through study of other groups of cannabis users is now needed.

It would also be of value to see whether the structural differences observed actually correlated with any demonstrable differences in thought processes and decision making behaviour.

As some US states have now, to all intents and purposes, legalised the sale of cannabis, such studies should be easier to carry out.

It is important to stress that cannabis has uncertain effects on thought processes, emotions and mental health, both in the short and longer term. It is also a class B drug which is illegal to possess or distribute.